1. Field of the Invention
This invention relates in general to magnetic sensors, and more particularly to a method and apparatus for reducing the thickness of a sensor stack in a current-perpendicular-to-plane (CPP) GMR/tunnel valve (TV) sensor.
2. Description of Related Art
Magnetic recording is a key segment of the information-processing industry. While the basic principles are one hundred years old for early tape devices, and over forty years old for magnetic hard disk drives, an influx of technical innovations continues to extend the storage capacity and performance of magnetic recording products. For hard disk drives, the areal density or density of written data bits on the magnetic medium has increased by a factor of more than two million since the first disk drive was used for data storage. Areal density continues to grow due to improvements in magnetic recording heads, media, drive electronics, and mechanics.
Magnetic recording heads have been considered the most significant factor in areal-density growth. The ability of the magnetic recording heads to both write and subsequently read magnetically recorded data from the medium at data densities well into the gigabits per square inch (Gbits/in2) range gives hard disk drives the power to remain the dominant storage device for many years to come.
Important components of computing platforms are mass storage devices including magnetic disk and magnetic tape drives, where magnetic tape drives are popular, for example, in data backup applications. Write and read heads are employed for writing magnetic data to and reading magnetic data from the recording medium. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
A magnetoresistive (MR) sensor changes resistance in the presence of a magnetic field. Recorded data can be read from a recorded magnetic medium, such as a magnetic disk, because the magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the read element, which causes a corresponding change in the sensor resistance.
A magnetoresistive (MR) sensor detects magnetic field signals through the resistance changes of a sensing element as a function of the strength and direction of magnetic flux being sensed by the sensing element. Conventional MR sensors, such as those used as MR read heads for reading data in magnetic recording disk and tape drives, operate on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy. A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the magnetic disk in a magnetic disk drive, because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance of the read element. This change in resistance may be used to detect magnetic transitions recorded on the recording media.
In the past several years, prospects of increased storage capacity have been made possible by the discovery and development of sensors based on the giant magnetoresistance (GMR) effect, also known as the spin valve effect. In a spin valve sensor, the GMR effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Magnetic sensors utilizing the GMR effect are found in mass storage devices such as, for example, magnetic disk and tape drives and are frequently referred to as spin valve sensors. In operation, a sense current is caused to flow through the read head and therefore through the sensor. The magnetic flux from the disc causes a rotation of the magnetization vector in at least one of the sheets, which in turn causes a change in the overall resistance of the sensor. As the resistance of the sensor changes, the voltage across the sensor changes, thereby producing an output voltage.
Recent hard disk drive designs have utilized the Current In-Plane (CIP) structure, where the sense current travels between the magnetic shields parallel to the sensor plate. Such a design has produced optimism that areal densities of 100 Gbits/in2 are possible, However, research efforts continue to find even better read heads so that areal densities may be boosted into the many hundreds of Gbits/in2 range.
One such discovery is the Current-Perpendicular-to-Plane (CPP) structure, whereby the sense current travels from one magnetic shield to the other, perpendicular to the sensor plate. The CPP head provides an advantage over the CIP head because as the sensor size becomes smaller, the output voltage of a CPP head becomes larger, thus providing an output voltage that is inversely proportional to the square root of the sensor area.
One of the candidates for realizing high sensitivity using the CPP structure is the Tunnel-Magneto-Resistive (TMR) head. In a TMR head, the magnitude of the tunneling current, in the gap between two ferromagnetic metals, is dependent upon the electron's spin directions or polarizations. Another candidate for the CPP structure uses a multilayer GMR structure that exhibits a large output signal.
The GMR sensor is continuously being scaled down to increase areal density. To meet targeted giant magneto-resistive (GMR) head aerial density requirements, designers are always looking for ways to make the sensor thinner, shorter and narrower. For example, if a CPP structure is used in-gap type read heads for high-density recording, the sensor films must be thinner than the read gap.
It can be seen that there is a need for a method and apparatus for reducing the thickness of a sensor stack in a current-perpendicular-to-plane (CPP) GMR/tunnel valve (TV) sensor.